Acoustic tweezers

ABSTRACT

Electroacoustic device having a transducer including a piezoelectric substrate, first and second electrodes of inverse polarity having respective first and second tracks provided on said substrate, the first and second tracks spiraling around a same center (C), the transducer being configured for generating a swirling ultrasonic surface wave in the substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This present application is a national stage filing under 35 U.S.C § 371of PCT application number PCT/EP2016/055611 filed on March 15, 2016. Thedisclosure of the above-listed application is hereby incorporated byreference in its entirety.

The present invention relates to electroacoustic devices notably formanipulating objects which size is less than 10⁻² m, immersed in aliquid medium and in particular being denser and/or more rigid than theliquid medium.

BACKGROUND

The selective manipulation of nano-sized and micro-sized objects is acomplex operation in various technical domains, such as cellularbiology, microfluidic, nano- and micro-sized system assembly.Manipulation might be performed using a tool, for instance tweezers or amicropipette. The object is then manipulated through displacement of thetool. Such a manipulating method, which is generally named “directcontact” method, is not desirable, in particular when the object issoft, or tacky, or even brittle. Furthermore, it may alter themanipulated object. Last, the introduction of the tool in a systemwherein the object is located may modify the properties of the system.For instance in case the object is submitted to an electromagneticfield, introducing the tool might create a disturbance of said field. Itcan also introduce some pollution. In case the system is a biologicalmedium comprising cells, the cell behavior can be modified by theintroduction of the tool.

Alternative contactless methods have been developed, such asdielectrophoresis, magnetophoresis, or optophoresis, also named “opticaltweezers” method. However, all these techniques have major drawbacks.For instance, dielectrophoresis depends on the object polarizability andrequires installing electrodes in the vicinity of the object to bemanipulated. Magnetophoresis requires grafting of markers onto theobject. Optophoresis may be used with or without grafting but is limitedto very small forces by the significant heating and photo-toxicityinherent of this method.

Another method has been developed, named “standing waveacoustophoresis”, which consists in implementing surface acoustic waves(SAW) generated in a substrate for manipulating an object lying oroverlapping the substrate.

U.S. Pat. No. 7,878,063 B1 describes an electroacoustic devicecomprising a substrate and three pairs of interdigitated transducers onthe substrate. Each pair of transducer defines an acoustic path forpropagating a surface acoustic wave generated by the transducers. Thethree acoustic paths intersect, thus creating a center region fordetecting biological species;

WO 2013/116311 A1 discloses an apparatus for manipulating particlescomprising a pair of variable frequency interdigitated transducers and achannel defined on a substrate, disposed asymmetrically between thetransducers.

WO 2015/134831 describes an acoustic apparatus including a firstinterdigitated transducer arrangement to generate a first acoustic waveand a second interdigitated transducer arrangement to generate a secondacoustic wave in a non-parallel direction relative to the first acousticwave, and a manipulation region at least partially defined by aninterference pattern at least partially formed by interaction betweenthe first acoustic wave and the second acoustic wave.

The article “Fast acoustic tweezer for the two-dimensional manipulationof individual particles in microfluidic channels”, S. B. Q. Tran, P.Marmottant and P. Thibault, Applied Physics Letters, American Instituteof Physics, 2012, 101, pp. 114103, describes a device comprising fourinterdigitated transducers provided on a substrate at a regular spacingaround a central zone. Each transducer generates a standing surfaceacoustic waves. Implementation of the device provides displacement of aparticle in the central zone.

US 2013/0047728 A1 teaches an apparatus comprising an ultrasound sourcefor providing a variable ultrasound signal within a region of interest,and a controller connected to the ultrasound source such that itprovides a control signal to the ultrasound source. The variableultrasound signal creates a pressure field within the region ofinterest, the shape and/or position of which can be altered by changingthe control signal input to the ultrasound source such that a particlewithin the region of interest will move in response to changes in thepressure field. However, the apparatus of US 2013/0047728 A1 isconfigured for generating bulk acoustic wave. As a consequence, itrequires components of large size which prevent from any use onlab-on-chips. In addition, it is not adapted to generate any surfaceacoustic wave.

All the known standing wave acoustophoresis methods consist ingenerating standing acoustic waves for manipulating objects. However,the selectivity of these methods is limited. In particular, all objectsdo move toward either the nodes or anti-nodes of the waves. As aconsequence, the standing wave acoustophoresis methods do not allow theselective manipulation of an object independently from its neighbors.

SOME EXAMPLE EMBODIMENTS

Therefore, there is a need for an electroacoustic device and for amethod for manipulating at least one object that overcome at least someof the drawbacks of the techniques of the prior art.

Exemplary embodiments of the invention relate to an electroacousticdevice comprising a transducer comprising a piezoelectric substrate,first and second electrodes of inverse polarity comprising respectivefirst and second tracks provided on said substrate, the first and secondtracks spiraling around a same center, the transducer being configuredfor generating a swirling ultrasonic surface wave in the substrate.

A swirling surface acoustic wave (SAW) is a wave that propagatesspinning around a phase singularity where destructive interferences leadto cancellation of the wave amplitude. A swirling SAW can propagate inan isotropic substrate and/or in an anisotropic substrate.

FIG. 1 illustrates the amplitude 5 of a swirling SAW at the surface ofan isotropic substrate along directions X and Y of the substrate. Aswirling SAW comprises an area 10 of low amplitude, generally named“dark spot” encircled by concentric rings 15 of high amplitude,generally named “bright rings”, illustrated in dashed line in FIG. 1 .The dark spot is an area of low radiation pressure whereas the brightcircles are zones of high radiation pressure. Therefore, a swirling SAWpropagating at the surface of a substrate is such that an object lyingfor instance on the substrate and located on a bright circle isattracted by the dark spot of the swirling SAW as indicated by thearrows 20 on FIG. 1 , as soon as its size is substantially equal orsmaller than the fundamental wavelength of the swirling SAW. The objectis entrapped by the dark spot.

The invention provides several advantages as compared to prior artdevices performing standing wave acoustophoresis. First, anelectroacoustic device according to the invention is simpler toimplement, since it can provide manipulation of an object with only asingle transducer. It may also be powered with a single low costpowering system. In addition, it does not require any specific settingof the transducer as compared to the prior art, where every transducerof the set of transducers has to be set precisely so that theinterferences of the SAWs generated by the transducers result in aradiation pressure field capable of object manipulation. Moreover, theinvention is not limited by any substrate property with regard to SAWpropagation. In particular, the substrate can be preferably anisotropic.Further, the electroacoustic device can be tuned to a wider range ofobject sizes than devices of the prior art. In particular, the devicecan apply larger forces than optophoresis devices on a same sized objectwithout destroying it.

In the present specification, a SAW is considered to have a frequencyranging between 1 MHz and 10000 MHz.

The electroacoustic device according to the first aspect of theinvention may further present one or more of the following optionalfeatures:

-   -   a set consisting in the first and second electrodes surrounds        entirely the center, and define a central zone;    -   the first track and/or the second track extend(s) over more than        90°, preferably over more than 180°, even preferably over more        than 270° around the center;    -   each of the first and second tracks spirals along a line defined        by the equation

${R(\Theta)} = \frac{\varphi_{0} - {\omega\;{\mu_{0}(\Theta)}} + {\alpha( {\overset{\_}{\psi}(\Theta)} )} - {\frac{\pi}{4}{{sgn}( {h^{''}( {{\overset{\_}{\psi}(\Theta)},\Theta} )} )}} - {l\;\Theta}}{\omega\;{s_{r}( {\overset{\_}{\psi}(\Theta)} )}{\cos( {{\overset{\_}{\psi}(\Theta)} - \Theta} )}}$

-   -   wherein:        -   R(θ) is the polar coordinate of the line with respect with            the azimuthal angle θ,        -   φ₀ is a free parameter,        -   l is the vortex order of a swirling SAW of pulsation ω, l            being an integer such that |l|≥1.        -   μ₀(θ) is given by:

${\mu_{0}(\Theta)} = {\sum\limits_{i = 1}^{n}{{s_{z}^{(i)}(\Theta)}( {z_{i} - z_{i - 1}} )}}$

-   -   -   where z_(i)−z_(i-1) is the distance between two successive            interfaces separating materials stacked onto the substrate,            z₀ being the height of the interface between the substrate            and the layer contacting the substrate, μ₀(θ)=0 in case of            the absence of stacked layers

${{h^{''}( \overset{\_}{\psi} )}\mspace{14mu}{is}\mspace{20mu}{\frac{\partial^{2}}{\partial\psi^{2}}\lbrack {{s_{r}(\psi)}{\cos( {\psi - \Theta} )}} \rbrack}\mspace{14mu}{evaluated}\mspace{14mu}{at}\mspace{14mu}\psi} = \overset{\_}{\psi}$

-   -   -   where ψ depends on Θ as follows:

${\overset{\_}{\psi}(\Theta)} = {\Theta + {{atan}\; 2( {\frac{s_{r}^{\prime}(\Theta)}{\sqrt{{s_{r}^{\prime\; 2}(\Theta)} + {s_{r}^{2}(\Theta)}}},\frac{s_{r}(\Theta)}{\sqrt{{s_{r}^{\prime\; 2}(\Theta)} + {s_{r}^{2}(\Theta)}}}} )}}$

-   -   -   s_(r)(ψ) is the wave slowness on the surface plane of the            substrate in the direction of propagation ψ, and s_(z)(ψ) is            the wave slowness in the out of plane direction, a wave            slowness in a direction i being r or z being computed from            the wavenumber k_(i) as s_(r)(ψ)=k_(r)(ψ)/ω: and            s_(z)(ψ)=k_(z)(ψ)/ω        -   s_(r)′(ψ) is the derivative of s_(r)(ψ) in respect to the            direction of propagation,        -   α(ψ) is the phase of the vertical motion of the wave            propagating in direction ψ versus the associated electric            field;

    -   the radial step between adjacent first and second tracks is        comprised between 0.48λ and 0.52λ, preferably equal to λ/2, λ        being the fundamental wavelength of the swirling ultrasonic        surface wave;

    -   each of the first and second tracks runs along at least one        revolution;

    -   the first and the second electrode comprise respective first and        second power terminals to which the first and second track are        electrically connected;

    -   the first and second electrodes comprise a plurality of        respective first and second tracks;

    -   the transducer is interdigitated;

    -   the electroacoustic device comprises first and second        transducers configured for generating first and second swirling        ultrasonic surface waves of different fundamental wavelengths in        the substrate, the first and second tracks of each of the first        and second transducers spiraling around a same center;

    -   the transducer intended to generate the lowest fundamental        wavelength among the first and second transducers surrounds the        other transducer;

    -   two consecutive first, respectively second tracks are separated,        along at least one radius, by at least two consecutive second,        respectively first tracks;

According to a second aspect, exemplary embodiment of the inventionrelates to an electroacoustic device comprising:

-   -   a piezoelectric substrate,    -   interdigitated portions of a set of at least four interdigitated        transducers arranged on said substrate around a central zone,        the transducers being configured for generating a surface        acoustic wave in said substrate,    -   a controller electrically connected to and providing electrical        power to each transducer and being configured such that the        surface acoustic waves emitted by the set of transducers        interfere the ones with the others to generate a swirling        ultrasonic wave at least in the central zone.

The electroacoustic device according to the second aspect of theinvention is particularly well adapted for generating swirling SAW in ananisotropic substrate.

Notably, the electroacoustic device according to this second aspect ofthe invention achieves generating a swirling SAW having a well-defineddark spot. It is thus more selective than the acoustophoresis devices ofthe prior art.

Furthermore, the electroacoustic device according to this second aspectis versatile, since the controller can be configured for generatingother kinds of SAWs, such as generalized Bessel waves of order l=0, oreven standing waves.

The electroacoustic device according to this second aspect of theinvention may further present one or more of the following optionalfeatures:

-   -   the electroacoustic device comprises at least eight, preferably        at least sixteen, or even at least thirty two interdigitated        transducers;    -   the interdigitated portions are substantially regularly arranged        around a central zone;    -   the interdigitated portions are arranged around a circle or        preferably around a line homothetic to the wave surface;    -   more than 50%, preferably more than 70% of the perimeter of the        central zone is defined by tracks of the transducers;    -   the interdigitated tracks of at least one, preferably all the        transducers, are curved;    -   the curved tracks are convex toward the central zone;    -   the curved tracks are concave toward the central zone.

According to anyone of the first and second aspects, the electroacousticdevice may further comprise one or more of the following optionalfeatures:

-   -   the transducer is covered by a protective coating, preferably        comprising silica;    -   the electroacoustic device further comprises a support        overlapping the transducer and the substrate, the support and        the substrate being acoustically coupled such that a swirling        ultrasonic surface wave generated in the substrate is        transmitted to the support and propagates as an acoustical        vortex or a degenerated acoustical vortex in the bulk of the        support;    -   the support is made at least partially of a non-opaque and        preferably transparent material;    -   the support is made of a non-piezoelectric material;    -   the support is made of an isotropic material with respect to the        propagation of an ultrasonic wave;    -   the support comprises a material chosen among a glass and a        polymer, in particular a thermoplastic, most preferably        polymethylmethacrylate (PMMA).    -   the support comprises glass;    -   the electroacoustic device comprises a layer made of a coupling        fluid sandwiched in between the substrate and the support;    -   the transducer is sandwiched in between the substrate and the        support;    -   at least a part of the substrate is sandwiched in between the        transducer and the support;    -   the transducer is configured for generating a swirling surface        acoustic wave such that the radius of the dark spot influence        zone of the swirling surface acoustic wave ranges between 0.1λ        and 0.7λ, preferably between 0.2λ and 0.55λ, λ being the        wavelength of the swirling surface acoustic wave; the “radius of        the dark spot influence zone” is defined by the distance between        the location of lowest amplitude in the dark spot and the by the        location of maximum of amplitude of the first bright ring;    -   the substrate is a plate having a thickness greater or equal        than 500 μm;    -   the electroacoustic device comprises a base, preferably made of        a non-piezoelectric material, on which the substrate is        disposed;    -   the base is made at least partially of a non-opaque, preferably        a transparent material, notably made of glass;    -   the substrate is in the form of a layer deposited onto the base,        the layer thickness being less than λ/10, λ being the        fundamental wavelength of the swirling ultrasonic surface wave;    -   the base is part of an objective of a microscope or is part of a        device configured to be fixed to an objective of a microscope;    -   the substrate is made of an anisotropic material, preferably        chosen among lithium niobiate, lithium titanate, quartz, zinc        oxide, aluminum nitride, lead titano-zircanate, and their        mixtures;    -   the substrate is at least partially made of a non-opaque,        preferably a transparent material;    -   the transducer is configured to generate a swirling surface        acoustic wave whose fundamental wavelength λ ranges between 10⁻⁷        m and 10⁻³ m;    -   the swirling surface acoustic wave is a generalized Lamb wave or        preferably a generalized Rayleigh wave;    -   the electroacoustic device comprises a plurality of transducers        configured for generating swirling ultrasonic surface waves of        different fundamental wavelengths in the substrate;    -   the electroacoustic device comprises a visual marking located in        the central zone of the transducer, preferably made of the same        material as the first and second tracks;    -   the electroacoustic device is disk shaped;    -   the substrate is mounted rotatable on a pivot around a rotation        axis    -   the electroacoustic device further comprises first and second        transducers, the location of the center of the first transducer        in a first arrangement of the device corresponding to the        location of the center of the second transducer in a second        arrangement of the device, the device being preferably        configured such that the transition from the first to the second        arrangement be operated by rotation around a pivot    -   the electroacoustic device comprises contact brushes for        powering the transducer electrodes;    -   the electroacoustic device comprises first and second        transducers, the contact brushes being in contact and powering        the respective first and second transducers, in respective first        and second arrangements of the device, the device being        preferably configured such that the transition from the first to        the second arrangement be operated by rotation around a pivot.    -   The electroacoustic device comprises an organ configured for        displacing the support relatively to the transducer, preferably        by translation along anyone of two axis both perpendicular, and        parallel to the substrate.

Preferably, the first and second electrodes are deposited onto thesubstrate by photolithography. In particular, a layer of a materialcomprising chromium or titanium might be deposited onto the substratebefore depositing the electrodes in order to improve the adherence ofthe electrodes on the substrate.

Preferably, the first and second electrodes are made from a metallicmaterial, preferably chosen among gold, silver, aluminum and theirmixtures. Aluminum is preferred for applications at frequency higherthan 100 MHz. Gold and/or silver are preferred when a good conductivityis required.

The width, measured along a radial direction of the tracks of the firstand second electrodes, can be equal. In a variant, the width can bedifferent.

The substrate can be plane or curved.

The present invention also relates to a method for configuring anelectroacoustic device according to the second aspect of the invention,the method comprising:

-   -   a. successively powering a single transducer i of the set of        transducers of the electroacoustic device with an electrical        input signal e_(i)(t), and if appropriate switching off the        other powered transducer(s), and measuring at each of several        control points j located on the central zone of the transducer,        the amplitude and phase the surface acoustic wave s_(j)(t)        generated in the substrate, and storing the measured surface        acoustic waves s_(j)(t)    -   b. computing the transformed signal input E_(i) and output S_(j)        of the respective input and output signals e_(i)(t) and        s_(j)(t),    -   c. computing the operator H_(ji) relating all the transformed        output signals S_(j) to all the transformed input signals E_(i)        through the relation S_(j)=H_(ji)E_(i)    -   d. determining the signal E′_(i) such that a transformed        swirling surface acoustic wave S′_(j) at point j is obtained by        the relation S′j=H_(ji)E′_(i)    -   e. computing for each transducer i the electrical input signal        e′_(i)(t) to be applied by inverse transformation of signal        E′_(i) such that a swirling surface acoustic wave swirls in the        central zone.

A “transformed signal” is obtained by a mathematical transformation thattransforms any convolution operation between two functions into a simpleproduct between these two functions. The mathematical transformation canbe chosen among the Laplace transform, the Z-transform, the Mellintransform and the Fourier transform. The Fourier transform is preferred.

Preferably, the method for configuring comprises storing the linearoperator H_(ji) and/or the amplitude and phase of each input signale′_(i)(t) in a storage unit, linked to or located inside the controller.

Exemplary embodiments of the invention also relate to an optical devicecomprising the electroacoustic device according to the invention.

The optical device according to the invention may further present one ormore of the following optional features:

-   -   the optical device is a microscope;    -   in at least one configuration of the optical device, the        transducer of the electroacoustic device is located between an        objective of the microscope and the support;    -   the electroacoustic device is fixed to an objective of the        microscope;    -   the optical device comprises a plurality of objectives, at least        two objectives having different magnifications, at least two,        preferably all the objectives being each fixed to an        electroacoustic device;    -   an electroacoustic device fixed on an objective of the plurality        is different from an electroacoustic device fixed on another        objective of the plurality.

The present invention also relates to a method for manipulating at leastone object in a liquid medium, comprising:

-   -   generating swirling surface acoustic waves with an        electroacoustic device comprising a transducer, and    -   propagating an acoustical vortex or a degenerated acoustical        vortex induced by said surface acoustic waves into the liquid        medium for creating therein a radiation pressure wherein said        object is submitted, and manipulating the object through        displacement of the transducer relative to the medium.

The method for manipulating at least one object in a liquid medium mayfurther present one or more of the following optional features:

-   -   the electroacoustic device is according to the invention;    -   the method comprises propagating the volume waves throughout the        bulk of a solid support before they reach the liquid medium;    -   the transducer is part of a device comprising on a single        piezoelectric substrate tracks of at least two respective        transducers, preferably interdigitated, having different        patterns of electrodes;    -   the device is rotatable about a rotation axis, and the method        comprises rotating the device before or after using the        transducer;    -   the method comprises displacing the transducer relative to the        medium using at least one electrical actuator;    -   the device comprises a visual marking located in the central        zone of the transducer, preferably made of the same material as        the first and second tracks, the method comprising arranging the        electroacoustic device such that the visual marking is offset        from the object, following by powering the transducer for        generating a volume ultrasonic wave in the liquid medium such as        to displace the object for it overlaps the visual marking;    -   the method comprises observing the object with an optical device        according to the invention;    -   the transducer comprises an array of electrode tracks, the        method comprising powering the electrode tracks with a single AC        source;    -   the method comprises inducing a hydrodynamic vortex in the fluid        in the vicinity of the object with an acoustical vortex or a        degenerated acoustical vortex for changing the orientation of        the object;    -   the method comprises generating acoustical vortices in the fluid        in the vicinity of the object in order to generate torques for        inducing rotation thereof;    -   the method comprises converging the acoustical vortex or the        degenerated acoustical into the liquid medium in a zone plumb        with the center of a central zone of the transducer, such as to        entrap the object along the direction around which the        acoustical vortex or the degenerated acoustical vortex swirls;    -   the method comprises inducing a hydrodynamic vortex in the        liquid medium;    -   the object is a biological material, preferably a cell, the        object preferably being label free;    -   the method comprises pre-distorting a wavefront of said        ultrasonic surface waves so as to control degeneration of the        bulk acoustical vortex, the pre-distortion preferably being        computed through an inverse filter method of the formula of        equation (1).

The present invention further relates to a method for manipulating atleast one object in a liquid medium or a liquid medium, notably being adroplet, according to a second aspect, the manipulating comprising atleast one of coalescing, deforming, mixing and aliquoting the at leastone object and/or the liquid medium, comprising generating swirlingsurface acoustic waves with an electroacoustic device comprising aplurality of interdigitated transducers powered by respective AC sourcesand propagating an acoustical vortex or a degenerated acoustical vortexinduced by said surface acoustic waves into the medium for creating apressure trap to which said object and/or said liquid medium issubmitted, and varying the AC sources for modifying the location andfeatures of the trap and manipulating, notably displacing or rotating,the object.

The method for manipulating at least one object in a liquid mediumaccording to the second aspect of the invention may further present oneor more of the following optional features.

-   -   the substrate is such that the ratio of the velocity of a wave        along the direction of maximum velocity of the substrate divided        by the wave velocity along the direction of minimum velocity of        the substrate is greatest than 1.3;    -   the electroacoustic device is according to the second aspect of        the invention;    -   the method comprises generating a hydrodynamic vortex with the        acoustical vortex or the degenerated acoustical vortex, notably        for mixing the liquid medium.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood from a reading of the detaileddescription that follows, with reference to exemplary and non-limitingembodiments thereof, and by the examination of the appended drawing, inwhich:

FIG. 1 illustrates the phase and amplitude of a 2D swirling SAW,

FIGS. 2 to 4 and 9 to 11 illustrate embodiments of an electroacousticdevice according to the first aspect of the invention,

FIG. 5 represents the amplitude of the vertical transverse displacementof a plane front wave in an anisotropic substrate depending on thepropagation direction,

FIG. 6 represents the Rayleigh velocity of a plane front wave in ananisotropic substrate depending on the propagation direction at theinterface with different media,

FIG. 7 is a 2D graph of the curve of φ₀−ωμ₀−1θ,

FIG. 8 is a 2D graph of the curve R(θ) used for defining the tracks ofthe electroacoustic device of FIG. 9 ,

FIGS. 12 and 14 represent electroacoustic devices according to thesecond aspect of the invention, and FIGS. 13 and 15 are respectivemagnified views of interdigitated portions of the transducers of FIGS.12 and 14 ,

FIG. 16 is a scheme illustrating the method of configuring theelectroacoustic device,

FIG. 17 illustrates measured and corresponding theoretical predictedphase and amplitude of a swirling SAW generated with an electroacousticdevice according to some embodiment of the invention,

FIGS. 18 to 23 show two variants of electroacoustic devices according tosome embodiments of the invention,

FIGS. 24 to 28 are pictures showing different objects manipulations, and

FIG. 29 is a mask for fabrication by photolithography of anelectroacoustic device.

In the drawing, the respective proportions and sizes of the differentelements are not always respected for sake of clarity.

DESCRIPTION OF SOME EMBODIMENTS

FIG. 2 illustrates an electroacoustic device 25 according to the firstaspect of the invention, comprising a substrate 30 and first 35 andsecond 40 electrodes of a transducer 43 disposed on the substrate. Thefirst and second electrodes comprise respective first 45 and second 50tracks which both spiral around a same center C.

The first and second tracks extend both over angles Ω₁ and Ω₂ greaterthan 270° around the center, but over different angular sectors. Theangles Ω₁ and Ω₂ may be equal or different.

The first and second electrodes comprise respective first 55 and second60 terminals for being connected to an electrical power supply 65. Thefirst and second tracks are connected to said respective terminals.

The terminals can be made of the same material as the electrodes andduring a same deposition process. As an alternative, they can be made ofdifferent materials.

The set consisting of the first and second tracks entirely surround acentral 70 zone comprising the center C, as shown in FIG. 1 . Thus,elementary SAWs are emitted by almost every angular section covered bythe first and second tracks, interfering to generate a swirling SAW inthe central zone.

The zone where the dark spot of the swirling SAW develops comprises thecenter C.

FIG. 3 illustrates an alternative embodiment which differs from the oneillustrated in FIG. 2 in that the first and second tracks run along morethan two revolutions around the spirals.

Increasing the number of revolutions results in an increase of theacoustic power of the swirling SAW.

The fundamental wavelength λ of the swirling SAW is determined by thedistance between two successive first and second electrodes. As shown inFIG. 4 , the radial step Δ between two consecutive first and secondtracks is preferably equal to λ/2.

In the electroacoustic devices illustrated in FIGS. 2 and 3 , the spiralis an Archimedes spiral, which is preferable in case the substrate ismade of a material which is isotropic as regard to SAW acoustic waves.

As it will appear hereunder, other shapes of the electrode tracks areadapted for propagating SAWs in anisotropic substrates.

Throughout the whole description, and unless stipulated otherwise, theterms “isotropy” and “anisotropy” respectively refer to isotropy andanisotropy with regard to the propagation of a SAW in any material.

In an anisotropic material, the generation of a swirling SAW is complex,since one has to deal notably with direction-dependent wave velocity,coupling coefficient and beam stirring angle. This can modify the waySAW propagating in different directions interfere.

In an anisotropic substrate, the wavelength of a SAW, its velocity andamplitude may depend on the direction along which the SAW propagates.

Furthermore, in case a material such as a support is stacked onto thesubstrate and is acoustically coupled with it, the swirling SAW can betransmitted in the bulk of support, but the SAW degenerates at theinterface between the substrate and the support in an acoustic vortex orin a pseudo acoustic vortex propagating in the bulk of the support. Theshape of the SAW, i.e. notably its phase and amplitude in differentsubstrate directions, is also modified by any isotropy mismatch betweenthe support and the substrate. The substrate may be made of ananisotropic material and the support of an isotropic material.

Preferably, each of the first and second tracks spirals along a linedefined by the equation (1):

${R(\Theta)} = \frac{\varphi_{0} - {\omega\;{\mu_{0}(\Theta)}} + {\alpha( {\overset{\_}{\psi}(\Theta)} )} - {\frac{\pi}{4}{{sgn}( {h^{''}( {{\overset{\_}{\psi}(\Theta)},\Theta} )} )}} - {l\;\Theta}}{\omega\;{s_{r}( {\overset{\_}{\psi}(\Theta)} )}{\cos( {{\overset{\_}{\psi}(\Theta)} - \Theta} )}}$where:

-   -   R(θ) is the polar distance coordinate of the line with respect        to the azimuthal angle θ. In other words it is a distance of the        spiral along a revolution at an angle θ and the center of the        spiral;    -   φ₀ is a parameter freely chosen to determine the center of the        spiral;    -   l is the vortex order of a swirling SAW of pulsation ω, l being        an integer such that |l|≥1.    -   ω=2πf is the fundamental angular frequency and f is the        fundamental frequency of the swirling SAW;    -   α(Θ) is the phase of the coupling coefficient of the        piezoelectric material constitutive of the substrate; for        instance, pure Rayleigh waves have a phase of 0, and pure        Gulyaev waves have a phase of π.    -   h(ψ, Θ)=s_(r)(ψ)cos(ψ−Θ) where s_(r)(ψ) is the phase slowness of        the swirling wave and is defined by s_(r)(ψ)=k_(r)(ψ)/ω,        k_(r)(ψ) being the norm of the radial component of the wave        vector at angle Θ;    -   the sign ′ denotes derivation on variable ψ;    -   function ψ(Θ) is defined by the equation

${{\overset{\_}{\psi}(\Theta)} = {\Theta + {{atan}\; 2( {\frac{s_{r}^{\prime}(\Theta)}{\sqrt{{s_{r}^{\prime\; 2}(\Theta)} + {s_{r}^{2}(\Theta)}}},\frac{s_{r}(\Theta)}{\sqrt{{s_{r}^{\prime\; 2}(\Theta)} + {s_{r}^{2}(\Theta)}}}} )}}};$and

-   -   the correction term μ₀ corrects the swirl degeneration in the        bulk of a stacking of a least one material acoustically coupled        with the substrate, when the swirling SAW is transmitted from        the substrate to the bulk of said material to propagate as an        acoustic vortex or a pseudo acoustic vortex; in order to        synthesize the precursor wave that will degenerate into a        swirling SAW at the desired height z_(n):

${\mu_{0}(\Theta)} = {\sum\limits_{i = 1}^{n}{{s_{z}^{(i)}(\Theta)}( {z_{i} - z_{i - 1}} )}}$

-   -   wherein s_(z) ^((i))(Θ)=√{square root over (s^((i)) ² (Θ)−s_(r)        ²(Θ))} is the phase slowness of the waves in each material (i)        of the stacking,

${s^{(i)}(\Theta)} = \frac{1}{c^{(i)}(\Theta)}$

-   -    being the phase slowness in the material (i) of the stacking,        c^((i))(Θ) being the wave celerity in the material at angle Θ,        and    -   z_(i)−z_(i-1) is the distance between two successive interfaces        separating materials stacked onto the substrate, z₀ being the        height of the interface between the substrate and the layer        contacting the substrate, and z₀ being the height of the surface        of the substrate wherein the swirling SAW is generated.    -   When no material is coupled with the substrate then μ₀(Θ)=0.

The position of a positive electrode track is defined by selecting theangle φ₀ in equation (1) and the position of the negative electrodetrack is then defined by the same equation (1) replacing φ₀ by φ₀=φ₀+π.

As it appears clearly in equation (1), although the pattern of a linearound which a track spirals can be adapted to a broad range ofsubstrate material and if appropriate to any support material stackedonto the substrate, it is nevertheless specific to a single set ofactuation frequency of the device, material properties and thicknesses.

In particular, the pattern shape relies on the frequency of the SAWpropagating in the substrate. In case material(s) are stacked onto thesubstrate so that a swirling SAW is transmitted and propagates in thevolume of these material(s) as an acoustic vortex or a pseudo acousticvortex, the pattern shape also depends on the velocities of the shearand longitudinal bulk acoustic waves in this (these) medium (media).

As shown in FIG. 5 , the amplitude 80 of a plane front SAW in ananisotropic substrate, for instance in a X-cut lithium niobiatesubstrate is dependent on the angle ψ of propagation of the wave in thesubstrate. The substrate anisotropy therefore affects the wavepropagation.

Furthermore, as shown by FIG. 6 , the Rayleigh velocity of a plane frontSAW at the surface substrate also depends on the direction ofpropagation of the wave. This dependence is observed whether thesubstrate surface is free and contacts air (curve 85) or a support, forinstance a 2 mm thick polymethylmethacrylate (PMMA) plate (curve 90) oreven a gold coating (curve 95) is disposed onto said substrate.

FIG. 7 is a graph representing the correction term φ₀−ωμ₀(Θ)−lΘ fordifferent values of angle Θ, as indicated along the periphery of thegraph. It is required for a swirling SAW swirls in an anisotropic X-cutlithium niobiate substrate and be transmitted and propagates in a 2 mmthick PMMA plate support acoustically coupled to the substrate. Values50, 100 and 150 along a direction at Θ=70° on the graph indicate,expressed in radians, of the correction term φ₀−ωμ₀(Θ)−lΘ.

FIG. 8 shows the trajectory of a line R(θ), expressed in mm, computedfrom equation (1) for a first order anisotropic swirl (l=1), and for ananisotropic X-cut lithium niobiate substrate. Angles expressed indegrees are regularly indicated at the periphery of the drawing. Thegraph of the line R(θ) takes into account the evolution of thecorrection term μ₀(Θ), the amplitude and Rayleigh velocity asillustrated on FIGS. 5 to 7 . In FIG. 8 , one observes at θ=45° a steeptransition due to a phase change for the first order swirling SAW.

FIG. 9 illustrates an electroacoustic device 25 comprising a transducer105 having first 35 and second 40 electrodes provided on a substrate 30and comprising a plurality of respective first 45 positive and second 50negative tracks. The tracks are provided on the X-cut lithium niobiatesubstrate following equation (1) described here above. The positivetracks are obtained considering an angle φ₀ in equation (1) and thenegative tracks are obtained by replace φ₀ in equation (1) φ′₀=φ₀+π.

Thus, the first and second tracks comprise the same center and aredistant along a radial direction D_(R) by a radial step equal to λ/2.

As it can be observed, the transducer is interdigitated. The first andsecond tracks are imbricated the ones with the others.

The electrodes comprise first 55 and second 60 power terminals havingthe shape of straight lines, which are respectively electricallyconnected to each of the first and second tracks.

For instance, the design of the tracks of the device of FIG. 9 followingequation (1) is adapted to generate a swirling acoustic wave in thesubstrate, and to propagate an acoustic vortex of wavelength equal to 10MHz in a 2 mm thick support made of PMMA provided on top of thetransducer, and coupled by a layer of silicon oil of a few micronsheight sandwiched in between the substrate and the support. The siliconoil layer achieves a coupling between the substrate and the supportwhile it does not affect substantially the propagation of the acousticwave since its thickness is much smaller than the acoustic wavelength.

The device according to the invention can be such that a set consistingin several tracks of the first electrode, in particular two tracks 110a,110 b as illustrated in FIG. 9 , running along a single first spiralwinding, and/or several tracks of the second electrode, in particulartwo tracks 115 a, 115 b as illustrated in FIG. 9 , running along asingle second spiral winding, surrounds entirely the center.

In addition, two adjacent first 110 b, 120, respectively second 115a,125 tracks can run along two consecutive winding of the first,respectively second spiral.

Furthermore, the first and/or the second power terminals and theplurality of first and/or second tracks of the device of FIG. 9 arearranged such that the first, respectively second electrode track, whenobserved along a direction normal to the substrate has a shape of afork.

A transducer as illustrated in FIG. 9 can be manufactured according tothe following method. A X-cut lithium 1 mm thick niobate substrate ispolished and cleaned, for instance with acetone-isopropyl-ethanol, andthen dried for 1 minute at 100° C. A layer of primer, and then ofAZ1512HS resin are deposited by centrifugation at 4000 rpm on asubstrate face and is annealed at 100° C. for 1 minute. A mask being thepositive of the pattern of the electrodes of the transducer is apposedon the resin. FIG. 29 illustrates a mask 126 for preparing anelectroacoustic device comprising a plurality of transducers as it willbe described latter. The primer is then exposed to an UV radiation. Thesubstrate is then placed in an evaporator in order to deposit a 50 nmthick chromium layer, followed by deposition of a 200 nm gold layer.

The substrate is then dipped into a bath of acetone submitted toultrasound emission at 80 kHz at a temperature of 45° C. for 10 minutes.

FIG. 10 represents an electroacoustic device comprising first 130 andsecond 140 transducers configured for generating first and secondswirling ultrasonic surface waves of different wavelengths in thesubstrate, the first and second tracks of each of the first and secondtransducers spiraling around a same center C. The first and secondtransducers share the same substrate 30.

The first transducer which is intended for operating at a lowerfrequency than the second one, surrounds the second transducer.

This specific configuration of the transducers results in a compactelectroacoustic device.

The substrate is the same and is oriented in the same direction as theone of embodiment of FIG. 9 .

The tracks of the first and second transducers are provided on thesubstrate both following respective lines of equation (1) as describedhere above. The parameters of equation (1) are chosen such that thefirst and second transducers generate a swirling SAW in the substrate atrespective fundamental frequencies of 10 MHz and 30 MHz, swirling aroundan axis passing through center C and perpendicular to the substrate,with respective first and second opposite spins.

First and second opposite spins are obtained by choosing respectiveappropriate swirl orders 1 of respective values +1 and −1 in equation(1).

The device illustrated in FIG. 10 is particularly well suited for anyapplication where the torque induced by the swirling SAW in the objectto be manipulated has to be controlled.

In particular, the track pattern of the electrodes is configured for anacoustic vortex or a pseudo acoustic vortex to be transmitted by thesubstrate and propagates into a 150 μm thick borosilicate glass sliceacoustically coupled with the substrate.

FIG. 11 illustrates an electroacoustic device 25 comprising a transducercomprising two sets 145, 150 of first and second electrodes. Thesubstrate 30 is the same as in examples of FIGS. 8 and 9 .

The first set 145 comprises first and second electrodes labeled 146 and148 and the second set 150 comprises first and second electrodes labeled152 and 154. Each of the first and second electrodes comprise first andsecond pluralities of tracks which follow a line of general equation 1.This electroacoustic device takes advantage that the order of the swirlis proportional to the frequency of the electrical input signal. Thefirst, respectively the second plurality of tracks spirals along a linewhich equation is computed considering a swirl order l equal to 1,respectively equal to 3.

Thus the transducer of the electroacoustic device illustrated in FIG. 11is adapted to generate signals operating at two fundamental frequenciesof 10 MHz and 30 MHz respectively.

In particular, the electroacoustic device is such that two consecutivefirst tracks along a radial direction are alternate in the radialdirection with two consecutive second tracks of the second electrode.

The device illustrated in FIG. 11 is notably well adapted for generatinga steady like current around the transducer center which makes itpossible to mix fluids or manipulate very small particles that would notbe trapped by a swirling SAW having only a single fundamental frequency.

FIG. 12 represents an electroacoustic device 160 according to the secondaspect of the invention.

It comprises a substrate 170, preferably an anisotropic X-cut lithiumniobiate crystal, having a central zone 175 which perimeter is delimitedby a circle 180 (illustrated in dash line on FIG. 12 ). Interdigitatedportions of thirty two unidirectional transducers 185 ₁ and 185 ₂ (onlytwo of them being labeled), preferably SPUDT interdigitated transducers,are provided onto the substrate around the central zone 175. Theinterdigitated portions 187 ₁, 187 ₂ are regularly spaced along thecircular perimeter of the central zone. The number of transducers is notlimited to thirty two. It is however preferred to be at least four. Ahighest number is preferred, since it provides a more uniform spatialcoverage of the central zone and enables a better synthesis of thetargeted wavefield. A high number of surface waves interfering improvethe generation of a swirling SAW.

As shown in FIG. 13 , each interdigitated portion 187 is composed ofinterdigitated first and second tracks of respective first 190 andsecond 195 electrodes of inverse polarity. At least one track of eachinterdigitated portion is tangent to the circular perimeter.Furthermore, as shown in FIG. 13 , the first and second tracks areparallel the ones with the others. The width of the first and secondtracks may be different or equal.

Each transducer further comprise first 200 and second 205 electrodeswhich comprise the tracks or the interdigitated portions as describedhereabove to which are connected respective first 210 and second 220power terminals. The first and second electrodes of each transducer areelectrically connected via the power terminals to a controller 225. Inthe drawing, for sake of clarity, only two sets of electrodes are shownas being connected but in practice, all the thirty two transducers are.

In the embodiment of FIG. 12 , the central zone is covered by a goldlayer 230 which is intended to serve as a mirror for measuring theamplitude and phase of the swirling SAW in the central zone with aMichelson interferometer.

In the electrical device of FIG. 12 , every transducer is configured forgenerating a standing SAW which propagates through the substrate, at afrequency that is of 12 MHz. The SAWs emitted by the thirty-twotransducers interfere in the central zone. As it will be more apparenthereafter, the controller is configured to control each of thetransducers such that the interference of the SAW generates a SAWswirling in the central zone.

FIG. 14 represents a variant of the electroacoustic device 170 of FIG.12 . It differs notably from the latter in that the perimeter 180 of thecentral zone 175 is not a circle but follows a line homothetic to thewave surface of a SAW of planar front propagating in the substrate.

The embodiment of FIG. 14 differs also from the one of FIG. 12 in thatthe first and second tracks are curved, as shown in FIG. 15 . Thecurving of the tracks promote wave diffraction, especially in ananisotropic substrate. This improves the illumination by each transducerof the central zone of the substrate, which has for example a radius of5 mm. This improvement allows to achieve a synthesis of a wide varietyof acoustic wave fields, such as focalized SAW, plane propagating SAWand notably of swirling SAW.

The specific curving of the tracks is performed following the teachingof the article “Subwavelength focusing of surface acoustic wavesgenerated by an annular interdigital transducer”, Laude et al., AppliedPhysics Letters 92, 094104 (2008).

The controller 225 of the device according to some embodiment of theinvention is configured to control each of the transducers such that theemitted SAWs interfere in the central zone to generate a swirling SAWtherein.

In particular, the controller powers each transducer by sending to it anelectrical input signal. Preferably, the controller comprises a storingunit wherein parameters of input signals to be sent to each transducerare stored. Preferably, the input signal is an AC electrical signal, andthe parameters are the maximum intensity and phase of the input signal.

Preferably, a method for configuring is implemented before the first useof the electroacoustic device, for instance such as shown on FIGS. 12and 14 .

This method for configuring, also named “inverse filter method”, isillustrated on FIG. 16 .

An electrical signal 230 e_(i)(t), preferably an impulse signal, is sentby the controller to a single transducer 235 i among the set oftransducers surrounding the central zone. The transducer converts thiselectrical input signal into a SAW which propagates into the centralzone 175. The controller is configured such that the electrical circuitsrelying the other transducers to the controller are opened. Thus noinput signal is sent from the controller to the other transducers.

The SAW emitted by transducer i in the substrate s_(j)(t) is measured ateach several control points j located in the central zone. Preferably,the number of control points 240 ₁,240 ₂ is at least 2, even preferablyat least 4, even preferably at least 10, even preferably at least 100,even preferably at least 200. As an illustration, 400 control points canbe distributed on a surface of 1×1 cm². Preferably the distance betweentwo control points is less than λ/2, λ being the wavelength of thestanding SAW emitted by transducer i. Preferably, the control points areregularly distributed in the central zone.

The amplitudes and phases of the SAW s_(j)(t) at all points j arepreferably measured with a Michelson interferometer 245, whose one armcan be focalized on any control point j. In case the substrate is madeof lithium niobiate, it is preferred to cover the central zone with agold layer which serves as a mirror to reflect the beams and improve thequality of the measurements.

After the input signal has been emitted, the controller switches off thetransducer i and sends an input signal e_(k)(t) to transducer 250 k. TheSAW s_(j)(t) are then measured at control points j.

The input signals e_(i)(t) of all successively powered transducers i andthe SAWs s_(j)(t) can be stored in a storing unit 255.

The input signals e_(i)(t) and measured amplitude and phase of the SAWss_(j)(t) can be related by the relationships _(j)=Σ_(i) h _(ij) *e _(j),

where * refers to the convolution product and h_(ij) is the timeresponse at control point j to an input signal e_(i) emitted bytransducer i. In the spectral domain, H_(ij)=

(h_(ij)) is the Fourier transform of the transfer function at controlpoint j of transducer i.

Using a matrix formalism, where E and S are vectors comprising therespective Fourier transforms E_(i) and S_(j) of signals. e_(i)(t) ands_(j)(t), and H is the matrix form of operator H_(ij), the linearfollowing relationship is obtained:S=H·E

Then, using well known classical pseudo matrix inversion techniques, avector E′ can be computed 260 for obtaining a vector S′ corresponding tothe Fourier transform of a Fourier swirling SAW at all control points j.

Finally, each component of the vector E′, which corresponds to theFourier transform of the input signal e′_(i)(t) to be emitted by eachtransducer i to generate a swirling saw can be obtained by inverseFourier transform 265.

Once the method for configuring is completed, the controller is thenconfigured for powering jointly several, preferably all the transducers,and for sending every transducer an output signal e′(t) 270. Each inputsignal has preferably its own features, such as specific phase and/oramplitude which are different between at least two emitting transducers.The interference in the central zone of the SAWs emitted by each of thetransducers thus generates a swirling SAW in the central zone of thesubstrate.

FIG. 17 illustrates the amplitude 275 ₁ and phase 280 ₁ of a first orderBessel wave swirling SAW generated with the electroacoustic device ofFIG. 12 , which has been set by the method for configuring according tothe invention. The amplitude and phase have been measured with aMichelson interferometer.

A dark spot 285 of 50 μm size is visible at the center of the swirl andmatches with a phase singularity. The dark spot is contrasted by brightconcentric rings. The theoretical amplitude 275 ₂ and phase 280 ₂ arealso represented for comparison. A correct matching between theoreticaland experimental swirls is achieved on both the amplitude and phase.

FIG. 18 illustrates an electrical device 300 according to someembodiment of the invention, comprising a support 305 overlapping thesubstrate 310. The support can overlap the electrodes 315 or it can onlyoverlap the central zone 320.

Furthermore, the support can be removable from the electroacousticdevice.

The tracks of the transducer can be located in between the substrate andthe support.

The support is preferably chosen among a glass and a polymer, preferablya thermoplastic, most preferably polymethylmethacrylate (PMMA).Preferably, the support is made of material comprising glass.

Preferably, the material of the support is isotropic. Preferably, it isnot piezoelectric.

In order to protect the tracks from friction by the support and preventfrom damage, the transducer is at least partially, preferably totallycovered by a protective coating 325, preferably comprising silica.Preferably, the protective coating thickness is less than λ/20, λ beingthe fundamental wavelength of the swirling SAW. Thus, the transmissionof the swirling SAW unaffected by the protective coating.

Preferably, for optimum transmission of acoustic waves, a coupling fluidlayer 330, preferably made of a silicon oil, is sandwiched in betweenthe support and the substrate. Preferably, the thickness of the couplingfluid layer is less than λ/20, λ being the fundamental wavelength of theswirling SAW. Thus, the transmission of the swirling SAW is unaffectedby the coupling fluid layer. Silicon oil is preferred since it has a lowdielectric constant and since it does not molder. Furthermore, thecoupling fluid allows easy displacement of the support relative to thesubstrate.

Electric brushes 335 are in contact with the electrodes for supplyingpower to the transducer.

As illustrated, the electroacoustic device can also comprise a cover 340provided onto the support, and comprising a groove 345 defining achamber, preferably made of PDMS, for instance having the shape of amicrochannel configured for housing a liquid medium comprising an object350 to be manipulated.

Preferably, in the embodiment of FIG. 18 , the swirling SAW is ageneralized Rayleigh wave. Preferably, the thickness of the substrate isgreater than 10λ, λ being the fundamental wavelength of the swirlingSAW.

As described previously, the pattern of the tracks of the electrodes canbe designed such that the swirling SAW generated at the surface of thesubstrate be transmitted and swirls 360 as an acoustic vortex or apseudo acoustic vortex in the support up to reach the liquid medium andthe object.

Preferably, in case the support is made of an isotropic material, thepattern of electrodes is such that the degeneration of the swirling SAWgenerated by the transducer at the interface between the substrate andthe support achieves an acoustic vortex or a pseudo acoustic vortex withan associated radiation pressure which concentrates in a volumerepresented as a square 365 located perpendicularly to the substrate andoverlapping over the center of the central zone of the transducer. Anobject located in the vicinity of said volume and having a sizecomparable to the wavelength of the swirling SAW, also named “3D trap”is submitted to attraction forces which aims at entrapping said objectin the volume. Notably, any displacement in the trap is limited, in allthe three space dimensions.

In a variant represented in FIG. 19 , the tracks of the electrodes canbe disposed on a face 370 of the substrate which is opposite to the face375 facing the support. Preferably, in the embodiment of FIG. 19 , theswirling wave is either a Lamb wave or a bulk wave.

In case it is a Lamb wave, the thickness if the substrate is lower thanλ/2, λ being the fundamental wavelength of the swirling SAW. Thissolution requires thinner substrates as the frequency increases.

Notably when the Lamb frequency would yield too thin a substrate, forinstance of thickness of less than 200 μm, the pseudo acoustic vortexcan be directly generated in a thicker substrate. It can be either abulk longitudinal wave pseudo acoustic vortex or a bulk shear waveacoustic vortex radiating in the thickness of the substrate at a fixedangle. The step between first and second tracks can be selected in orderto match with the projection of the wavelength.

Advantageously, in the embodiment of FIG. 19 , the transducers areprotected from any damage by the support are from any pollution causedby the coupling fluid. Furthermore, the face of the substrate which iscontact with the support can be easily cleaned without any risk ofdamaging the electrodes, when the support is removed from the substrate.A device having tracks provided on the face opposite to the support asin FIG. 19 can comprise a high conductive coupling fluid of a highdielectric constant coupling fluid, such as a water based gel, withoutthe coupling fluid influencing negatively the swirling SAW generationand propagation.

Furthermore, the electrical connections, such as contact brushes can beprovided on the same side as the tracks, which simplifies themanufacturing of the device, and makes it more ergonomic to the user.

FIG. 20 describes a variant of the electroacoustic device 380 accordingto the first aspect of the invention which comprises a substrate 385,which is disk shaped of center C_(D). The substrate comprises aplurality of electrode patterns 390 ₁, 390 ₂ provided on the substratedefining a plurality of transducers 395 ₁, 395 ₂. Preferably, asillustrated, the transducers are regularly provided around the center ofthe disk.

The electroacoustic device further comprises a support 400 which ispreferably non opaque, and more preferably transparent. The supportpartially overlap the substrate. The support and the transducers areprovided such that in at least one position of the device, at least oneof the transducer is entirely overlapped by the support. Preferably, asillustrated in FIG. 18 , the tracks are provided on the face of thesubstrate that is intended to face the support.

A cover 403 is disposed on the support.

The substrate is provided rotatable around an axis X_(D) passing throughthe center C_(D) of the disk. In particular, the electroacoustic deviceis configured such that, by rotating the substrate around axis X_(D),each transducer among the plurality of transducer can be positioned suchas to be overlapped by the support and, notably by an object to bemanipulated provided on the support.

Moreover, as illustrated, the electroacoustic device can comprise amicro-manipulator 405, connected to the support, which allows for aprecise positioning by translation of the support relative to atransducer, preferably along two perpendicular axes preferably parallelto the substrate. The micro-manipulator can be fixed to an opticaldevice such as a microscope.

Furthermore, the electroacoustic device comprises outer 410 and inner415 contact brushes for electrically powering the electrodes. It canalso comprise a power supply device 420 to which the contact brushes canbe electrically connected. Preferably, the ends 425, 430 of the contactbrushes intended for contacting the electrodes can be fixed with regardto the substrate. In particular, they can be provided at a constantpolar coordinate relative to center of the substrate.

Each electrode of the plurality comprises a first 435 ₁, 435 ₂ andsecond 440 ₁, 440 ₂ power terminal. All the power terminals of theelectrodes of a same polarity are preferably provided radially on a sameside of each transducer. As illustrated in FIG. 20 , the power terminalsof the respective first and second electrodes of the transducers arerespectively radially outside and inside of the tracks of the electrode.In addition, all power terminals of the first electrodes areelectrically connected to a common power track 450, which extends,preferably around a circle, at the periphery of the substrate.

The outer contact brushes are preferably in contact with the externaltrack. By the way, when the user of the device rotates the substratesuch as to place a specific transducer such as it faces the support, theelectrical contact between the first electrode and the outer contactbrush of said transducer is achieved with no move of the outer contactbrush.

Preferably, each of the second power terminals of one of the transducersis provided such that, when the substrate is rotated around the axisX_(D) in order that the transducer faces the support, the second powerterminals is in electrical contact with the inner contact brush.

Advantageously, the electroacoustic device illustrated in FIG. 20requires a single power supply device and a single contact brushes pairto successively power each transducer. It does not require any complexcontrol system with expensive electronic devices and is thereforecheaper than electroacoustic devices of the prior art. In addition, asdescribed here above, manufacturing of the electrical device comprisingseveral transducers can be performed by photolithography which issubstantially inexpensive, for instance with the mask 126 as illustratedin FIG. 29 .

Furthermore, the device is easy to use, since the user can select anytransducer of the device by a simple rotation operation. Besides, as itcan be observed on FIG. 20 , each transducer is visible by the userwhich facilitates its initial positioning prior to manipulation of anobject.

As a matter of illustration, FIG. 21 shows an electroacoustic device 460which differs from the one of FIG. 20 by the fact that the electrodetracks are disposed on the face 465 of the substrate 470 opposite to theone that faces the support, as already illustrated in FIG. 19 .

FIG. 22 shows a crop of a microscope 480 comprising the electroacousticdevice 380 of FIG. 20 . The electroacoustic device is fixed onto themicroscope deck, such that a zone of the support, on which an object tobe manipulated is disposed, overlaps an objective 485 of the microscope.

The optical device allows observation of an object 490 trapped in thecentral zone 495 while being manipulated by the electroacoustic device.

In the variant of FIG. 23 , the transducer 500 of the electrical deviceof the invention is disposed on an objective 505 of the optical device.As objective magnification is directly related to the size of the objectintended to be manipulated, the transducer disposed on the objective ispreferably adapted to manipulate an object which can be entirelyobserved with the objective. Preferably, a single transducer is disposedon the objective.

The transducer can be provided on the outer lens, notably the protectionlens of the objective. It can also be provided in an inner lens of theobjective. Preferably, the substrate of the electrical device is in theform of a coating made of a piezoelectric material (such as AlN, ZnO)deposited on the objective, preferably having a thickness related to thefrequency used by the electrical device to optimize the generationefficiency, on top of which electrodes are disposed, preferably beingdeposited by photolithography. The objective may comprise means forpowering the transducer.

In a variant, the substrate can be disposed on a base which isconfigured to be fixed to the lens. The base can comprise a part made ofa non-opaque, preferably transparent material on which the substrate isdeposited as a layer.

Preferably, a coupling fluid is sandwiched in between the objective andthe support.

In the embodiment of FIG. 23 , a swirling SAW generated by thetransducer can be propagated, and be transmitted as an acoustic vortexor a pseudo acoustic vortex for instance through an immersion oilwherein the lenses of the objective are embedded.

In a preferred embodiment, the optical device comprises theelectroacoustic device according to the first aspect of the invention.

The embodiment as exemplified in FIG. 23 makes the optical device morecompact and manipulation of the object is made easier. Further, itreduces issues related to light propagation which might be encounteredin substrates having a thickness of greater than 1 mm.

Furthermore, the optical device can comprise a plurality of objectives,each objective comprising an electroacoustic device according to theinvention, the electroacoustic devices being different the ones from theother. Preferably, each transducer has a pattern of electrodes whichdiffers from the pattern of electrodes of at least, preferably all thetransducers of the plurality. For instance, it is thus possible tosuccessively change the objective of the plurality such as to trap anobject in respectively smaller and smaller traps.

The electroacoustic device, for example comprised in an optical devicesuch as the microscope as illustrated in FIG. 22 , might be used, forinstance, as follow:

A user can dispose a liquid medium comprising an object on top of thesupport. Then, he may firstly position the liquid medium as to beoverlapped by the field of view of the objective, for instance bytranslating the support with the micro-manipulator.

Then he might choose the transducer which is adapted for the intendedobject manipulation, for instance chosen among displacement, mixing,coalescing and aliquoting. As described previously, the fundamentalfrequency of a swirling SAW is defined by the electrode patterns of thetransducer. A man skilled in the art knows how to choose an appropriatefrequency depending on the size of the object to be manipulated.

The user might then rotate the substrate such that the object and thesupport overlap the chosen transducer. With the micro-manipulator, theuser might then position a visual marker 515 indicating the position ofthe center of the transducer, such as illustrated for instance in FIG. 9, with regard to the support and the object. The visual marker alsopreferably corresponds to the position of the dark sport of the swirlingSAW, on top of which the object is to be entrapped.

Then, by powering the transducer, and generating a swirling SAW which istransmitted and propagates as an acoustic vortex or a pseudo acousticvortex in the support up into the liquid medium, the object ismanipulated, displaced and trapped on top of the dark spot.

EXAMPLES 1 TO 4: DISPLACEMENT, FUSION, ATOMIZATION AND DIVISION

A water droplet of initial volume 2 μl is disposed on the central zoneof the electroacoustic device illustrated on FIG. 14 . Burst of duration25 μs, carrying a frequency of 11.9 MHz and variable repetition rates ofa several kHz are used for droplet actuation. The corresponding pulsesequences are presented on the bottom of FIGS. 24 to 27 .

For every type of the pulse sequence 520 illustrated on FIG. 24 to 27 ,the following manipulations have been performed: droplet displacement525 (FIG. 24 ), fusion 530 of two droplets (FIG. 25 ), dropletatomization 535 (FIG. 26 ) which are obtained respectively with swirlingSAWs of second and zero order. FIG. 27 shows droplet division 540 whichis obtained by synthesizing burst of focalized waves with two differentfocal points.

EXAMPLE 5: CELL MANIPULATION

Manipulating of cells and droplets are performed with the microscope asillustrated in FIG. 22 , such as to create homogeneous or heterogeneousnetworks of cells such as stem cell niche, made of stem cells havingsimilar physical properties.

Droplets are the basis of droplet-based microfluidics, used in thedomain of single-cell biology. The electroacoustic device of theinvention allows an in-depth study of rare events by sampling themwithin a large pool of experiments, currently a major issue of cancerand drug resistance research.

In this view, a central zone of a transducer is placed under a set ofparticles to be manipulated by displacement provided by themicro-manipulator. When a particle is at the center of the central zoneof the transducer, the power supply is turned on to generate a swirlingSAW in order to submit the particle to the attraction effect of the darkspot of the SAW. Operating is performed with a swirling SAW having afrequency of 30 MHz, and with voltage amplitude of 5 Vpp, which areenough such to entrap 10 μm sized particles.

Then the support is moved by translation provided by themicromanipulator while the trap, i.e. the position of the particlerelative to the center of the transducer, remains fixed in space,whereas the other particles which are remote from the trap follow thesupport translation.

Once the selected object is moved, electrical power is turned off.

Then the procedure is repeated for displacing another particle such asto gather particles in a predefined pattern.

The trapping force is proportional to the acoustic power and isinversely proportional to the wavelength. It is also stronger forobjects whose density and/or elasticity deviates from the fluid medium.

EXAMPLE 6: CELL DEFORMATION

The electroacoustic device is also implemented to apply forces onbiological cells and particles.

It is nowadays understood that forces and stress on cells may determinetheir fate. Somatic cells adapt to stress and may rigidify, and stemcell differentiation may be affected by external mechanical stress.Nevertheless, methods were limited to apply stress on cells.

A liquid medium comprising antibody-coated microspheres and a cellmembrane is placed beneath the object to be manipulated by displacementprovided by the micro-manipulator. A suitable transducer is electricallypowered in order to entrap the antibody-coated microspheres on top ofthe center of the transducer. While electrical power is applied, thesupport is displaced such that the cell membrane comes into contact withthe antibody-coated microspheres and is deformed by said microspheres.

EXAMPLE 7: STEADY CURRENTS AND VORTICITY

Swirling SAWs are generated to create a steady swirling current in amicrochannel, which is useful for contactless mixing, or for applyinghydrodynamic stress to, or for moving particles of size of less thanλ/10.

The streaming velocity is proportional to the acoustic power in amedium, and it increases with anyone of the square of the wavefrequency, the swirl order, and the square of the height of the channel.

A chamber having a groove defining a microchannel is placed on thesupport, the groove being located plumb with the transducer center. Aliquid medium having a set of particles is placed in the microchannel.

The groove has a depth preferably larger than λ, λ being the wavelengthof the swirling SAW. Powering the transducer results in streamingobserved in the microchannel, in the form of a cyclone formed in theliquid medium, its eye being located at the center of the radiatingswirling SAW. In order to promote streaming, the frequency might beincreased, for instance using another transducer.

EXAMPLE 8: PARTICLE DISPLACEMENT

A droplet comprising a suspension of fluorescent polystyrene beads 550of diameter 30 μm is deposited on the support of an electroacousticdevice as illustrated on FIG. 20 . A 3 mm thick cover made of PDMS isprovided on top of the droplet, and defines an acoustic absorber as wellas a slice glueing the droplet. A bead is chosen and is placed on topand close to the visual marker, and the transducer is powered. Then, thesupport is displaced by the manipulator, and the transducer is switchedoff, leaving the selected bead in a defined position. Repeating thisoperation on other beads of the suspension, a pattern of aligned beadsdefining the word “LIFE” is obtained, as illustrated in FIG. 28 .

Needless to say, the invention is not limited to the embodimentssupplied as examples.

The present invention is also notably intended for applications in thedomain of microscopy, biology, microfluidics, for lab-on-chips, formanipulating nano- and micro-systems. In biophysics, it can be used forstudying the behavior of single cells such as cancer cells or stemcells, and of cells networks, for instance implied in Alzheimer illness.

The invention claimed is:
 1. An electroacoustic device comprising atransducer comprising a piezoelectric substrate, first and secondelectrodes of inverse polarity comprising respective first and secondtracks provided on said substrate, the first and second tracks spiralingaround a same center, the transducer being configured for generating aswirling ultrasonic surface wave in the substrate, wherein each of thefirst and second tracks spirals along a line defined by the equation${R(\Theta)} = \frac{\varphi_{0} - {\omega\;{\mu_{0}(\Theta)}} + {\alpha( {\overset{\_}{\psi}(\Theta)} )} - {\frac{\pi}{4}{{sgn}( {h^{''}( {{\overset{\_}{\psi}(\Theta)},\Theta} )} )}} - {l\;\Theta}}{\omega\;{s_{r}( {\overset{\_}{\psi}(\Theta)} )}{\cos( {{\overset{\_}{\psi}(\Theta)} - \Theta} )}}$wherein: R(θ) is the polar coordinate of the line with respect with theazimuthal angle θ, φ₀ is a free parameter, l is the vortex order of aswirling SAW of pulsation ω, l being an integer such that |l|≥1. μ₀(θ)is given by:${\mu_{0}(\Theta)} = {\sum\limits_{i = 1}^{n}{{s_{z}^{(i)}(\Theta)}( {z_{i} - z_{i - 1}} )}}$where z_(i)−z_(i-1) is the distance between two successive interfacesseparating materials stacked onto the substrate, z₀ being the height ofthe interface between the substrate and the layer contacting thesubstrate, μ₀(θ)=0 in case of the absence of stacked layers${{h^{''}( \overset{\_}{\psi} )}\mspace{14mu}{is}\mspace{20mu}{\frac{\partial^{2}}{\partial\psi^{2}}\lbrack {{s_{r}(\psi)}{\cos( {\psi - \Theta} )}} \rbrack}\mspace{14mu}{evaluated}\mspace{14mu}{at}\mspace{14mu}\psi} = \overset{\_}{\psi}$where ψ depends on Θ as follows:${\overset{\_}{\psi}(\Theta)} = {\Theta + {{atan}\; 2( {\frac{s_{r}^{\prime}(\Theta)}{\sqrt{{s_{r}^{\prime\; 2}(\Theta)} + {s_{r}^{2}(\Theta)}}},\frac{s_{r}(\Theta)}{\sqrt{{s_{r}^{\prime\; 2}(\Theta)} + {s_{r}^{2}(\Theta)}}}} )}}$s_(r)(ψ) is the wave slowness on the surface plane of the substrate inthe direction of propagation ψ, and s_(z)(ψ) is the wave slowness in theout of plane direction, a wave slowness in a direction i being r or zbeing computed from the wavenumber k_(i) as s_(r)(ψ)=k_(r)(ψ)/ω: ands_(z)(ψ)=k_(z)(ψ)/ω s_(r)′(ψ) is the derivative of s_(r)(ψ) in respectto the direction of propagation, α(ψ) is the phase of the verticalmotion of the wave propagating in direction ψ versus the associatedelectric field.
 2. The electroacoustic device according to claim 1,wherein the radial step (Δ), between adjacent first and second tracks iscomprised between 0.48λ and 0.52λ, λ being the fundamental wavelength ofthe swirling ultrasonic surface wave.
 3. The electroacoustic deviceaccording to claim 1, wherein each of the first and second tracks runsalong at least one revolution.
 4. The electroacoustic device accordingto claim 1, wherein the first and second electrodes comprise a pluralityof respective first and second tracks and/or the transducer isinterdigitated.
 5. The electroacoustic device according to claim 1,comprising first and second transducers configured for generating firstand second swirling ultrasonic surface waves of different fundamentalwavelengths in the substrate, the first and second tracks of each of thefirst and second transducers spiraling around a same center.
 6. Theelectroacoustic device according to claim 1, further comprising asupport overlapping the transducer and the substrate, the support andthe substrate being acoustically coupled such that a swirling ultrasonicsurface wave generated in the substrate is transmitted to the supportand propagates as an acoustical vortex or a degenerated acousticalvortex in the bulk of the support.
 7. The electroacoustic deviceaccording to claim 1, comprising a base, on which the substrate isdisposed.
 8. The electroacoustic device according to claim 7, whereinthe base is part of an objective of a microscope or is part of a deviceconfigured to be fixed to an objective of a microscope.
 9. Theelectroacoustic device according to claim 1, comprising a visual markinglocated in the central zone of the transducer.
 10. The electroacousticdevice according to claim 1, further comprising first and secondtransducers, the first and second transducers being mobile between afirst arrangment and a second arrangement, the location of the center ofthe first transducer in the first arrangement of the devicecorresponding to the location of the center of the second transducer inthe second arrangement of the device.
 11. The electroacoustic deviceaccording to claim 1, wherein a set consisting in the first and secondelectrodes surrounds entirely the center and define a central zone. 12.The electroacoustic device according to claim 1, the first track and/orthe second track extend(s) over more than 90° around the center.